Recently, gas engines that operate on a natural gas serving as a fuel have been being developed and introduced in order to cut the amount of emission of nitrogen oxide (NOx) or to reduce the amount of emission of CO2.
As examples of the problems to be addressed in order to bring the gas engine operating on a natural gas into practical use, may be mentioned the following: that the compositions of natural gases may be different depending on where the gases are produced; and that the composition of a fuel gas may vary by the amount of consumption of the fuel gas being varied at the start of the gas engine or when variation in load occurs. A change in the composition of a fuel gas would lead to a change in the properties of the fuel gas such as the calorific value or the methane number, thereby causing an abnormal combustion such as engine knocking or misfire. Here, the methane number is an indicator which indicates the resistance value to knocking corresponding to the octane number of a gasoline engine and which is evaluated as 100 for pure methane and 0 for hydrogen.
In order to avoid such an abnormal combustion, it is considered as effective means to acquire the fuel properties of the fuel gas such as the calorific value or the methane number in real time so as to provide combustion control to the gas engine on the basis of the data.
For example, the present applicant has suggested a method of measuring the calorific value of a fuel gas such as a natural gas, the method including: measuring, for example, a physical property value having a particular correlation with the calorific value; and determining the value of calorific value on the basis of the measurement value (converted calorific value) (refer to Patent Literature 1, for example).
On the other hand, mainly employed as a method of calculating the methane number of a fuel gas are the following four types:
(a) A scheme suggested by AVL (hereafter, also referred to as “the AVL Standard”);
(b) A scheme of calculating by a particular arithmetic expression specified by California Air Resources Board (hereafter, also referred to as “the CARE Standard”);
(c) A scheme of calculating by a method in conformity to ISO/TR 22302 3.1.1 (hereafter, also referred to as “the GRI (Lc) Standard”); and
(d) A scheme of calculating by a method in conformity to ISO/TR 22302 3.1.2 (hereafter also referred to as “the GRI (H/C) Standard”). Here, the methane numbers of the same fuel gas may exhibit different values depending on the calculation methods, and for example, a methane number based on a different standard by area is demanded. However, in any of the methods, it is necessary to measure the gas composition in calculating the methane number when a variation occurs in gas composition, since the methods are to calculate the methane number on the basis of the gas composition, as described above.